The invention relates to an emergency brake assistant for automatically decelerating a vehicle to prevent a collision or reduce the consequences of a collision according to the preamble of Claim 1.
Modern driver assistance systems are capable of completely preventing an imminent collision or at least minimizing the consequences of a collision by an autonomously initiated full brake application. By means of a suitable sensor system (radar, lidar, image processing) or the evaluation of vehicle-to-vehicle communication, these systems detect the vehicle environment and determine possible collision objects. When a collision is imminent, a full deceleration will be initiated.
With an increasing relative speed between the given vehicle and the potential collision partner, the path required for this braking will be increased quadratically. Since, in comparison, the distance for a narrow overtaking maneuver or evading maneuver increases only in a linear fashion with the speed, at higher differential speeds, an overtaking or evading will still be possible after the point in time of a collision-preventing braking has already been exceeded. This so-called intervention dilemma results in a target conflict for the design of the emergency brake assistant. If, when the last-possible braking point in time is reached, the assistant intervenes by means of a full deceleration, the collision will be prevented. A driver, who had planned a narrow overtaking or evading maneuver, will, however, be surprised by the intervention. In order to prevent these decelerations subjectively perceived to be faulty and meet the product liability requirements, the systems currently on the market are designed such that a braking does not take place before the point in time at which the driver can neither by braking nor by overtaking or evading still autonomously prevent the collision (“point of no return”). However, at high differential speeds, this has the result that, also by means of the autonomous emergency braking, the preventing of a collision is no longer possible—only the consequences of the collision will be reduced.
From German Patent Document DE 601 26 398 T2, a brake control system with a system intervention in the event of an object recognition is known, which, when the driver's intention to evade is recognized, suppresses an otherwise generated automatic braking intervention for preventing a collision with a detected collision object.
It is an object of the invention to indicate an emergency brake assistant which, while taking into account the above-mentioned problems, can determine an intervention point in time so that, on the one hand, the driver feels sufficiently safe and, on the other hand, is not hindered when carrying out his own driving action.
This object is achieved by means of a emergency brake assistant according to Claim 1. Advantageous further developments are contained in the dependent claims.
The invention is based on an emergency brake assistant for automatically decelerating a vehicle to prevent a collision or reduce the consequences of a collision with a detected collision object, wherein, at a determined intervention point in time, a brake system of the vehicle is automatically triggered such that a collision with the detected collision object is prevented or the consequences of the collision are at least reduced.
In real road traffic, the driver will constantly experience events to which he has to react to in order to prevent a collision—these include the following application cases relevant to an autonomous emergency braking:                A continuous approach to a vehicle ahead,        a sudden braking maneuver of a vehicle ahead as well as, and        an adjacent vehicle cutting into the given traffic lane.        
Although no information concerning the subjective driver's perception is available, a model of these reaction triggers can be created by means of the vehicle sensor system. For this purpose, driving-psychological threshold values are used which describe the transition from a following to an approaching that requires a reaction.
Continuously approaching a vehicle within a given trajectory, which in many situations takes place at a high relative speed, can also be described independently of the distance by the time to collision (TTC). In the state of the art, a TTC value of 5 seconds is considered as a threshold below which the driver feels an acute need to act for again increasing the gap between his vehicle and the other vehicle or for evading the obstacle by means of an overtaking maneuver.
In a limited differential speed range, this threshold value can also be indicated as a distance value. It is composed of a constant safety distance when stopped as well as a speed-dependent value. By means of this minimal following distance, the cases of “sudden braking” and “cutting-in” can now be detected. If a vehicle driving ahead were to decelerate in traffic so much so that a following driver of a given vehicle would fall below the safety distance while still moving, a requirement to act would arise for restoring the safe driving condition. This need to act also arises when a vehicle in the adjacent lane starts a cut-in maneuver within the minimal following distance.
Both events would also be recognizable by means of the time-related threshold value TTC. However, the advantage of considering the distance is exhibited by the example of a sudden braking. If the deceleration of the vehicle ahead starts in a driving condition in which it is still moving away from the given vehicle, the computation of the TTC would not yet be possible. By using the distance value, the starting point in time for the beginning of the subsequent reaction can be identified also in such cases.
The invention is characterized in that the intervention point in time can be determined as a function of the end point in time of a determined driver reaction time and of the determined last-possible braking point in time.
The reaction time, which starts after the occurrence of the corresponding reaction trigger and lasts until the point in time at which the driver reaction can be measured on the vehicle bus system, can be modeled by means of the OODA activity sphere. This sphere represents the individual components of human decision-making: O(bserve), O(rient), D(ecide), and A(ct).
The observation process as well as the activity process can be based on approximately constant values averaged for all drivers. The duration of the mere perception of the situation is approximately 0.2 seconds; the execution of the act—thus, the movement of the foot for the actuation of the pedal—because of frequent practice, takes approximately 0.3 seconds.
In contrast, the remaining process steps for the classification, as well as for the decision-making, are dependent on the triggering event; i.e., the driver's reaction time can be determined as a function of the current vehicle environment and/or of the type of the occurrence of the collision object. Unexpected triggers (for example, unpredictable strong braking of the vehicle ahead) or events with a low probability of occurrence lead to a longer phase of decision-making than expected or frequently occurring events. Together, these two process steps can be modeled to be gamma-distributed.
In order to be able to derive a reaction time from the gamma distribution, the risk of exceeding a certain duration is considered. For this purpose, the cumulated gamma distribution is subtracted from one. Accordingly, a risk of 20% means that, in the considered examination, 80% of the drivers required a shorter or precisely the resulting duration for the reaction to the triggering event.
In addition to the dependence on the triggering event or on the type of the occurrence of the collision object, the reaction time resulting from the OODA activity sphere will increase if several acting alternatives are available to the driver. Since the individual occurrence probabilities of the acting alternatives are unknown a priori, they are assumed to be evenly distributed.
For the application case of active danger braking, this means that, in addition to the possibility of braking, the feasibility of an overtaking maneuver also has to be examined. For this purpose, for example, the area adjacent to the vehicle is, in each case, divided into three sub-areas: Behind the given vehicle, laterally at the level of the given vehicle, and in front of the given vehicle. For all areas, a value relevant to a lane change is computed and, in a comparison function, is standardized to a factor between 0 (lane change not feasible) and 1 (lane change safely feasible). The minimum of the three factors determines the global lane change feasibility. If a defined threshold value for the minimum is exceeded, it is assumed that a lane change into the corresponding direction is feasible.
In the rear area, the necessary deceleration for an approaching vehicle in the event of a lane change of the given vehicle, is computed as a value relevant to the lane change. In the front area, the necessary deceleration for the given vehicle is analyzed should the latter carry out a lane change and the target lane already be occupied by another vehicle. In the lateral area, the space required for making the lane change is analyzed. The lane change feasibility is then obtained from the ratio between the necessary and the reasonable deceleration (rear and front area) or between the necessary and the available space in the lateral area.
Since, particularly when assuming that in the event of a detected collision object, the driver can possibly still overtake or evade, the last-possible evading point in time has to be taken into account when determining the intervention point in time, this last-possible evading point in time is also taken into account in an advantageous further development of the invention.
For determining the last-possible evading point in time or the necessary distance for a narrow overtaking maneuver, for example, an overtaking parabola with an assumed lateral deceleration is placed in the free area between the given vehicle and the potential collision object. The distance in the longitudinal direction necessary for the overtaking or evading maneuver is then obtained as a function of the momentary relative speed, as well as the lateral distance to be overcome. The latter may consist of the widths of the given vehicle and the other vehicle/object as well as of the current object position. Since a maximal lateral acceleration cannot be assumed at low speeds, the lateral acceleration is adapted as a function of the speed.
Taking into account the end point in time of the determined driver reaction, a last-possible braking point in time determined (in a known manner) and the determined last-possible evading point in time, the situation can be evaluated to the end of the reaction time, and thereby at least a preliminary intervention point in time can be determined for an autonomous braking intervention. For this purpose, as indicated above, the corresponding reaction time is first assigned to the three possible reaction triggers. A continuous approach to a vehicle ahead is, in this case, considered to be an expected event; a sudden braking or a cutting-in vehicle is considered to be an unexpected event. If, at the point in time of the occurrence of the reaction requirement, the corresponding reaction duration is subtracted from the time to collision (TTC), the determined value—thus, the time after the reaction that remains for preventing the collision—can be compared with the threshold values for a braking or an overtaking or evading maneuver.
In the case of low differential speeds (i.e. here, with respect to the time, the last-possible evading point in time is before the last-possible braking point in time), the last-possible braking point in time can always be used as an intervention point in time, because the driver has no other alternative for preventing the collision.
In the case of high differential speeds, at which an overtaking or evading is still possible after the last-possible braking point in time, however, three possible situations will occur.
In the first case, if with respect to the time, the determined end point in time of the driver reaction time is before the determined last-possible braking point in time, the last-possible braking point in time can therefore be selected as the preliminary intervention point in time because, in the case of an autonomous collision avoidance by the driver, a corresponding reaction already exists beforehand. For this case, the avoidance of an accident will therefore always be possible.
In the second case, it is assumed that the determined end point in time of the driver reaction time occurs before the determined last-possible evading point in time and after the determined last-possible braking point in time. Since the probability of an autonomous mastering of the situation by the driver is much higher than that of a rear collision, a braking intervention, if necessary, will be initiated only at the end of the reaction time. However, in the least favorable case, this may mean that the emergency braking will no longer be sufficient for completely preventing the collision. In order to partially solve this conflict, the standardized value for judging the lane change possibility from the reaction time estimation will be used; i.e. in this case, the preliminary intervention point in time is defined as a function of the feasibility of an evading maneuver. In this case, the last-possible braking point in time is defined as the preliminary intervention point in time if an evading maneuver is not feasible, while the determined end point in time of the driver reaction time will be defined as the preliminary intervention point if an evading maneuver can be carried out safely. If no lane change is possible, a fully collision-preventing braking will therefore take place.
In the third case, it is assumed that the determined end point in time of the driver reaction time is after the determined last-possible braking point in time and after the determined last-possible evading point in time, the determined last-possible evading point in time occurring after the determined last-possible braking point in time. In this case, the automatic braking intervention may already be carried out at the last-possible braking point in time because a timely reaction by the driver is very improbable.
For an even better definition of the intervention point in time of the emergency brake assistant, it is also important, in addition to the above-mentioned parameters, to check and take into account the actual driver reaction. In particular, the driver reaction (for example, braking or overtaking) occurring during and/or after the determined driver reaction time should be taken into account. If there is a reaction (or it is at least believed that there is a reaction) and it is sufficient for the autonomous avoidance of the collision, or the evading or take-over maneuver can be carried out, the preliminary intervention point in time determined in the differentiation of cases will be delayed to the so-called “point of no return”, thus the determined last-possible evading point in time. Although the latter should never be reached because of the driver reaction that was judged to be adequate, it can nevertheless be maintained as a fall-back level. Otherwise, the determined preliminary intervention point in time is defined as the intervention point in time.
Indicators can in each case be determined for the two action options “braking” and “overtaking”. A braking effect can be applied by the normal operating of the brake pedal as well as by the buildup of drag torque by releasing the gas pedal. The overtaking reaction can be detected by recognizing a decreasing overlap with respect to the vehicle ahead or by a corresponding course of the steering angle. For an early inclusion of the intention to overtake, the use of lane change motivation models is obvious. The recognition of the flasher operation for the lane change is a simplified approach for this purpose.
For checking whether the detected reaction is also present in a timely and adequate manner, in the case of the braking, the applied deceleration is compared with the deceleration required for preventing the collision. The latter is a function of the momentary relative speed between the given vehicle and the collision object, the momentary distance between the two vehicles as well as the deceleration of the other vehicle.
In the case of an existing intention or reaction for the purpose of overtaking, it has to be checked—as mentioned above—whether the lane change or the evading maneuver is feasible. The evaluation of the change-over possibility computed within the scope of the reaction time estimate can be used for this purpose.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.